Of course, a minute later, when the AP confirmed that the tweet
was false, the stock market bounced right back to where it was
trading before the tweet hit the stream.

Société Générale FX strategist
Sebastien Galy writes in an email to clients that this
episode "will prove a good test of the positioning of
the markets."

"You now have the sensitivity of the markets to a
large negative (fictitious) shock from which to deduce the
positioning of the market," says Galy.

The most notable moves seem to be the S&P 500, which
instantly fell 1% on the news, the VIX, which jumped 9%, and the
dollar-yen exchange rate, which instantly dropped 0.7% (meaning
the dollar weakened against the yen).